Surfactant deficiency and injury is an important component in the pathogenesis of acute lung injury (ALI)and acute respiratory distress syndrome (ARDS). Analysis of bronchoalveolar lavage (BAL) samples from ALI/ARDS patients reveal multiple abnormalities including decreased total surfactant phospholipid with a disproportionately greater decrease in phosphatidylglycerol (PG). PG is critical to maintain optimal surface activity through its interactions with surfactant protein B (SP-B). Secretory phospholipases A2 (sPLA2) are potent mediators of surfactant injury, and are increased in the BAL fluid in ALI/ARDS. Depletion of PG is a key mechanism in sPLA2-mediated surfactant dysfunction. Identification of specific sPLA2 and mechanisms regulating secretion are unknown in ARDS. This proposal will examine the hypothesis that specific secretory phospholipases A2 released during the inflammatory phase of ALI and ARDS hydrolyze pulmonary surfactant phospholipids, decrease PG and play a major role in limiting endogenous surfactant function and clinical recovery from acute respiratory failure. This hypothesis will be tested as follows: Specific Aim 1. Measure bronchoalveolar phospholipase A2 and surfactant abnormalities during the course of ALI/ARDS. BAL fluid will be obtained from ALI/ARDS patients (n = 80) early in the course of respiratory failure and serial samples will be obtained over the course of respiratory failure. Components of the BAL (cells, surfactant pellet and supernatant) will be analyzed and the associations with important clinical variables of ARDS etiology, respiratory failure and ARDS outcome will be established. Specific Aim 2. Determine the mechanisms regulating sPLA2 expression and secretion. sPLA2 proteins secreted into the BAL will be identified and the regulation of sPLA2 synthesis and release from cell models measured in vitro to test the hypothesis that there is a cytokine driven stimulation of sPLA2 secretion from specific cells in the lung. Cytokine analysis of the BALs in Aim 1 will be used to further direct these studies. Specific Aim 3. Determine the mechanisms that regulate sPLA2-mediated surfactant hydrolysis and dysfunction. The impact of the surfactant proteins (SP) and the alveolar phospholipid forms (bulk aggregates in the subphase and monomolecular films at the air-liquid interface) on sPLA2 hydrolysis, PG depletion and surfactant dysfunction will be examined. Interpretation of the in vivo relevance of these results will be enhanced by measuring SP levels in the BALs from Aim 1. This multicenter, multidisciplinary analysis directly addresses the NHLBI ALI Workshop priority to investigate biochemical predictors of ALI, and is essential to develop effective clinical intervention studies involving suppression of sPLA2 activity and surfactant replacement.